1. Field of the Invention
The present invention relates to a method for determining the minimum in-situ stress of an underground formation and, more particularly, to such a method which may be performed quickly, accurately, and without the necessity of specialized tools.
2. Setting of the Invention
Most producing oil or gas formations require some form of secondary recovery method after the production has decreased beneath a certain level. One such recovery method is by water drive where fluid is injected into producing formations to force the oil towards a production well. Pressure at which the fluid is injected is very important in the control of secondary and/or tertiary recovery projects. In certain situations it has been found that for best control, the fluid should be injected at a rate and pressure below the fracture parting pressure (also referred to as the minimum in-situ stress, fracture closure pressure or fracture gradient). This minimum in-situ stress is also important in the design and analysis of hydraulic fracturing stimulation projects.
In the past, for enhanced recovery operations, the in-situ stress has normally been determined by a "step-rate" test. In these tests the fluid injection rate is increased in small increments or steps and the resulting injection pressure is measured. At a certainrate, a plot of the pressure versus rate will show a decreasing slope, or the injection rate can increase with little or no increase in pressure. The pressure where this change in slope occurs is termed the fracture parting pressure or the in-situ stress. In actual practice, this procedure has not been very satisfactory because the test is time-consuming, and the data is often ambiguous.
Also, the inferred parting pressure is likely to be the fracture extension pressure (pressure to extend a fracture) which is greater than the pressure to open a fracture. The pressure to open the fracture is generally the desired pressure level for operations. A second procedure, which is used in fracture design work, involves straddle packers used in an openhole section. "Mini-breakdowns" are then pumped between the packers to measure the in-situ stress. This procedure provides accurate test results; however, this procedure requires an unfractured open hole and the test is subject to mechanical problems, such as packer leaks.